My hip flexors still kill me sometimes although not as much as they used to. For a while I thought it was possibly due to my form. I think I have finally come to the conclusion that it is from sitting at a desk in front of computer 8 hours a day coupled with my 1 hour commute each way, that I am always going to have messed up hip flexors.
I don't think hip flexor strength is my issue. I've got D. Coaching vid of me on here squatting 360lbs x 5 and somehow I just don't think my hip flexors can't keep up. That's like saying your biceps are sore from pullups so you need to work on your curls. That doesn't make sense. I just assume keep working my pullups and watch my bi's grow with my back.
Also, I have read articles that say just the opposite for people who have a desk job. The articles I have read say to work the crap out of all the opposite muscles, i.e. posterior chain, to counteract the effects of sitting all day. They don't say anything about doing more hip flexor work.